Cutting pistons for lower compression with a wood router

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
Just what she says on the tin.
Pistons are getting expensive, and motors are getting pretty doggone cheap. Every ALH I take apart has a bent #2 and 3 from being hydrolocked after cleaning out the intake schmutz. Maybe the "weak" alh rods are just because they were already bent to begin with. We'll see.
So, round up two sets of rods to put together a passable set, stick a bent rod in your bench vise as a fixture to hold your nicer pistons, find you a $20 craigslist wood router and go to town.

Edge trim bit, for laminate countertops. This'll delip the pistons nicely.

An ALH piston appears, lonely, scared, unaware of what is about to happen

Boop, goes quick.

Leaves a knife edge of flash down in the bottom where the bearing can't ride flat on the radius, but a carpenter's knife whittles out the alu just fine so the roundover bit's bearing will ride where it needs to.

Some turn out nice, when you spend some time and hold it tight

Others turn out pretty nasty, when the router rocks over and takes it out further than the guide bearing should allow. Totally my fault, didn't have near the care that I did when I did my ARLs a while back, those turned out nice...

You'll notice some got the poke cut off, one didn't. I weighed them before and after cutting, so that all would get their combustion chambers increased in size the same amount, for an even compression ratio. Since I did a crap job at cutting them on the guide bearing, I had to enlarge the ones I did a nicer job at not wallowing out badly. All are within a gram (which is a surprisingly large amount of aluminum) as that's all my scale's resolution's good for.
Going to reuse the rod bolts, because they look fairly okay with a moment's inspection. Likely well within safety margin for a second use, but not a third or fourth.
Block's been honed out for .004" piston/wall clearance, so the pistons can expand as they get hot. Plan is a 4T65E set up for 2.55:1 final drive with a torsen from a 5hp19fla, an HX40 QSV'd, 12mm/de110 pump and likely another set of DSS-r .360s because then I can get a calibrated pump voltage map worked up for both my engines at once. Got the trans mostly sorted, adapter plate partially drawn up and partially cut out, flexplate mostly cut up... Slow going, but that's life now that I'm working full time.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
Interesting project. Curious as to how the combustion process will have changed with the reshaping of the bowls. Have you looked at a CR piston to see how they are shaped? I know being a 16v engine they may be very different anyway, but those do enjoy a lower compression ratio.
 

Yucca

Veteran Member
Joined
Apr 13, 2007
Location
Finland
TDI
ALH 388bhp, Polo 6R 2.0TDI CR GTC1752VZ
Too much advance is only problem for those Rod failures. I have not seen any bended rod if software is from good tuner.

We have many setup between 350-400bhp with stock rods. Max. advance is between 20-22 degree (after 5000rpm!).
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
Interesting project. Curious as to how the combustion process will have changed with the reshaping of the bowls. Have you looked at a CR piston to see how they are shaped? I know being a 16v engine they may be very different anyway, but those do enjoy a lower compression ratio.
I've already done a set of ARLs, without needing to adjust bowl volume cutting out the center bit because I was a bit more careful. Those are in my... only car with plates at the moment, for the last 8-10 tanks of fuel.

they seem to run fine, slightly more cold white smoke, but that's just to be expected running lower compression, big injectors and no glowplugs (they normally run them after it's running for a short while when it's real cold until the pistons have a chance to warm up a little bit)
Too much advance is only problem for those Rod failures. I have not seen any bended rod if software is from good tuner.

We have many setup between 350-400bhp with stock rods. Max. advance is between 20-22 degree (after 5000rpm!).
Kinda figured it was a combination of too much timing, and little tiny turbos that let you make 800 n/m at 2500 rpm if you don't use the torque limiting maps.
 

Markus L

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Feb 12, 2015
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Caddy (14D) 1.9TDI 2-stage 400+bhp, Passat 3BG 4motion AVF
Too much advance is only problem for those Rod failures. I have not seen any bended rod if software is from good tuner.

We have many setup between 350-400bhp with stock rods. Max. advance is between 20-22 degree (after 5000rpm!).
22deg @ 6000rpm is ok if you like big EGT and low power.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
A little WD40 does wonders for cutting aluminum at wood suface speeds. Stops that welding and sticking and greatly improves the surface finish.
nah, it was cutting great with a mirror finish when climb-cutting rather than conventional milling.

I was just flapping the router around without a care in the world after the first one got a nice gouge from rocking the router up off the piston top.
 

Jukums

Well-known member
Joined
Jan 28, 2014
Location
Latvia
TDI
1.9 TDI 1Z TDR.lv
I also have made piston bowls bigger, but I am not statisfied with it, because in cold time under 20 degrees celsius, it smokes a lot with white smoke, even when engine is warm. So I don't know is it good idea and if it make any power gain at all.
 
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Yucca

Veteran Member
Joined
Apr 13, 2007
Location
Finland
TDI
ALH 388bhp, Polo 6R 2.0TDI CR GTC1752VZ
I also have made piston bowls bigger, but I am not statisfied with it, because in cold time under 20 degrees celsius, it smokes a lot with white smoke, even when engine is warm. So I don't know is it good idea and if it make any power gain at all.
Bigger bowls does not make power, you have to match your fueling levels, boost and timing for new situation.
 

Jukums

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Joined
Jan 28, 2014
Location
Latvia
TDI
1.9 TDI 1Z TDR.lv
SOI is changed, but theres isn't possible to make car don't smoke with white smoke at idle, when driving the white smoke appears only in low RPM when turbo isn't making any boost. SOI at idle are 7bdtc, and 23-24 till end 7k rpm.
 

mrchill

TDIClub Enthusiast, Super Secret Diesel Ninja Vend
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Sep 16, 2003
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MASS! home of THE WORLD SERIES CHAMPION RED SOX! x
TDI
96 B4v red \ 98 Mk3 green\98 Mk3 Jetta black\ 99 Mk4 Jetta green x2\ 99 Mk4 Golf silver x2\ 99 Mk4 Jetta black\ 97 B4 sedan green\04 JSW gold\03 JSW silver
The safe limit on a VE is 17:1. Anything lower will have some cold smoke. The lowest I have tested on a VE is 15.6:1. I got the smoke to be very minimal except below 20 degrees F. It sounded like a gas car. Almost no diesel clatter at all.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
I also have made piston bowls bigger, but I am not statisfied with it, because in cold time under 20 degrees celsius, it smokes a lot with white smoke, even when engine is warm. So I don't know is it good idea and if it make any power gain at all.
https://uploa][/QUOTE]

Yeah, it is losing power because your expansion ratio decreases. The gain is that you don't lift the cylinder head with lots of fuel.

ETA: why are your head studs full diameter rather than being turned down to the minor diameter of the threads on the ends? It is hard for some to believe, but you'll gain some strength by doing this.
[QUOTE="Jukums, post: 5450060, member: 408763"]SOI is changed, but theres isn't possible to make car don't smoke with white smoke at idle, when driving the white smoke appears only in low RPM when turbo isn't making any boost. [/QUOTE]
it's just when combustion temperature is too low to reliably burn all fuel
run it up 5-6k RPM in 2nd gear, no throttle, your EGTs will be something nuts like 100 degrees C so of course there is fuel that does not burn

I've thought about overadvancing the timing at light load so the engine runs against itself to get some heat going on but haven't tried it yet.
You can see it in the stock maps where in the very cold SOI maps the low iq high RPM cells have much more advance than even high iq, high rpm. They do that so that it loads down the engine and makes heat.
 
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[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
volume went... up
I didn't even write down the difference in mass, I think it was something like 8 grams?
could figure the volume from the density of alu and go from there, but really this motor doesn't warrant such meticulous calibration. I reused the main and rod bolts.
You don't gain strength, but you gain fatigue resistance. A stud threaded over its entire length will stretch evenly over its entire length. Minimal stress concentration and less fatigue at the ends of the thread. Take a look at a stock head bolt. Necking it down will accomplish the same thing. Stock bolts have rolled threads over their entire length, so the cross section is consistent throughout.
I don't talk out loud often, so my word choice is... ****

just like turning down axleshafts to the minor diameter of the smallest spline. Makes the twist spread out over the whole axle rather than concentrating mainly where the axle's smaller. Makes it more likely to spring back after being stressed rather than remaining twisted (and eventually failing after a couple similar loadings).
 
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Votblindub

Veteran Member
Joined
Dec 22, 2010
Location
NY
TDI
MK4 Jetta Wagon
volume went... up
I didn't even write down the difference in mass, I think it was something like 8 grams?
could figure the volume from the density of alu and go from there, but really this motor doesn't warrant such meticulous calibration. I reused the main and rod bolts.

I don't talk out loud often, so my word choice is... ****

just like turning down axleshafts to the minor diameter of the smallest spline. Makes the twist spread out over the whole axle rather than concentrating mainly where the axle's smaller. Makes it more likely to spring back after being stressed rather than remaining twisted (and eventually failing after a couple similar loadings).
I'm curious about this motor and have a few questions. Somebody told me de-lip my pistons, hang a second turbo off it and up the boost some more.
 

joep1234

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Jan 2, 2014
Location
NC
TDI
former '04 Beetle TDI, now 2x '15 Audi Q5 TDI's, 2007 Dodge Ram 4x4 6.7
Interesting idea. Having come from the old gas motor world, when you change the piston design, weight or head relief, they have to be cc'd so they match from cylinder to cylinder so that the power will be balanced. I know that the bowl needs to be smooth. If there is a bur, it can be an ignition spot thus throwing off your timing. Hope it works for you in the long haul. I'll be following your post.
 
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[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
Interesting idea. Having come from the old gas motor world, when you change the piston design, weight or head relief, they have to be cc'd so they match from cylinder to cylinder so that the power will be balanced. I know that the bowl needs to be smooth. If there is a bur, it can be an ignition spot thus throwing off your timing. Hope it works for you in the long haul. I'll be following your post.
diesels are kind of cool in a few ways when running them balls out and doing your own tuning

there is no possibility of preignition as all that's in the cylinder before injection is... air
bad displacement balance may skew your AFR a little bit cyl/cyl, but that doesn't really matter as this most helpful thing compared to gasoline:
'rich' AFR only makes for smoke, no actual damage
'lean' AFR only reduces power (as that's how they're governed) from peak

I know about the troubles with tuning gas motors to stay together and they're worlds harder to get right than diesels.
I'm curious about this motor and have a few questions. Somebody told me de-lip my pistons, hang a second turbo off it and up the boost some more.
Yeah, go for it. Tons of... Fun? Keeps you busy for a while at least. Just ask all the guys here with compounds. :p
 

Digital Corpus

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Joined
Mar 14, 2008
Location
Ontario, California
TDI
'97 B4 w/ 236K mi body, 46K mi soul
Changing bowl geometry has been tried and talked about on here before, though [486]’s approach is novel and better than just removing the lip
 

p0wer

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Joined
May 21, 2009
Location
Jyvaskyla, Finland
TDI
Golf 3 -94 1Z 377hp, Golf 3 Cabrio 4-Motion 1.8T 620hp, Golf 3 Syncro 2.9 VR6 HX52, Bora AJM 4-Motion 2260vk +120% Firad, Passat 3B Syncro AFN 2260vk
[QUOTE='[486];5450128'
I've thought about overadvancing the timing at light load so the engine runs against itself to get some heat going on but haven't tried it yet. [/QUOTE]


This helps, done it for years alredy. It's helpful with even only bigger (0.3+) injectors. I have 4-5 degrees on idle and i think 7 degrees on very light load on my lowered cr, 0.341mm nozzled engine.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
I think mine's set at like 7-8 degrees on the timing belt pulley, then tops out around 15 degrees in the maps at high speed and load, the .360s still smoke white with de110/12mm pump when it is very cold.
I was thinking about calling for 10 or more degrees at idle, but maybe that would be too much and there wouldn't be enough heat for ignition that early. I'll have to try it some time when I'm reflashing the maps and see.
 
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